It is already known that when aluminum thin film is used as wiring material, the direction of the crystal particles and crystal particle diameter affect deterioration, e.g., wire breakage due to electromigration (abbreviated EM hereafter).
In short, the life of aluminum wiring until it disconnects because of EM (EM resistance): MTF (mean time to failure) is expressed by the following formula, based on experience, of S. Vaidya. EQU MTF.alpha.(S/.sigma..sup.2) log (I.sub.111 /I.sub.200).sup.3 . . . (1)
Where, S: aluminum polycrystal grain diameter PA1 .sigma.: variation in this crystal grain diameter PA1 I.sub.111 : (111) face X-ray diffraction intensity PA1 I.sub.200 : (200) face X-ray diffraction intensity
According to the formula (1), as the crystal grain diameter increases, variation becomes smaller, and the fact that crystal grain orientation (phenomenon where crystal direction follows a specific direction) is controlled by the (111) face crystal direction is important for improving EM resistance.
Prior to this, aluminum wiring has used a construction, as shown in FIG. 30, where it is applied to contact hole 4 of insulation layer 3 on semiconductor substrate 1 as a single layer film 12 and conducts on insulation layer 3 by connection to a specific semiconductor region 22. With this type of single layer aluminum wiring 12, however, aluminum atoms diffuse toward the semiconductor substrate during annealing or heating, forming an Al-Si alloy. Spike alloy 10 shown in FIG. 30 is readily produced, and if this reaches pn junction 11, there will be a problem with shorting between wiring 12 and substrate 1. Because of this, even if the aluminum crystal diameter is controlled as described above, EM resistance will be insufficient.
So, as shown in FIG. 31, barrier metal film 13, to prevent aluminum atom diffusion, is provided beneath aluminum film 12, and the formation of wiring with this stacked structure having aluminum film 12 and barrier metal film 13 is widely used.
The use of titanium nitride (TiN), for example, as the barrier metal film 13 is already known. Along with the barrier function of TiN, Al crystal orientation is controlled by controlling the TiN crystal orientation. In other words, this takes advantage of certain characteristics. By controlling TiN crystal orientation to (111), Al crystal orientation will follow the (111) face orientation. With TiN and Al made of the same face centered cubic construction: FCC, the lattice constants of the two are comparatively close, 4.2417 .ANG. for TiN and 4.0494 .ANG. for Al, and the Al (111) crystal face will grow in conformity to the TiN (111) crystal face.
This TiN/Al stacked film, has the problems shown in (1)-(6) below, however.
(1) TiN is applied by reactive sputtering or Ti is nitride-treated after sputtering. In either case, as shown in FIG. 32, when the TiN applied by sputtering or Ti 13 is particularly thin where it adheres to the inside of contact hole 14, defects occur readily, and the material in this state cannot function as an underlying layer (barrier metal) for the aluminum film applied as the top layer. If the film is made thicker, it will adhere together above contact hole 4, as indicated by the imaginary lines, completely blocking contact hole 4. With the top aluminum film unable to adhere inside contact hole 4, contact defects can easily occur. In either case, since sputtering is used, there is a tendency for step coverage to be poor, and conditions for uniform application are limited.
(2) Since the work function difference between TiN and n-type silicon and p-type silicon is very large, electrical contact of TiN film 13 alone to a silicon substrate (semiconductor region 22 above) is difficult. To improve this, a metal film, Ti, etc., must be used as an underlying layer for TiN film 13. Thus, even when TiN is applied only to contact hole 4, i.e., so-called selective formation (selective-TiN), electrical contact will not be obtained.
(3) In addition, even when TiN is formed from contact hole 4 on insulation film 3, as a so-called blanket type, electrical contact is not obtained with TiN alone.
(4) TiN film 13, which serves as the aluminum film underlying layer, has the same (111) crystal plane orientation as the aluminum, so crystal direction may be over-aligned. With annealing or heating, aluminum atoms readily diffuse toward the semiconductor substrate along the TiN crystal grain boundaries through the TiN film, and a spike alloy as described above may be created.
(5) Controlling TiN crystal orientation to the (111) face requires special conditions. That is, for film formation conditions, TiN is deposited by controlling substrate bias during reactive sputtering, or Ti deposited by sputtering is nitrided by lamp illumination in N2. In addition, depending on the degree of nitriding, reproducibility of the crystal orientation may be insufficient.
(6) The problems as described in the aforementioned (1)-(5) paragraphs occur not only when TiN is applied to the contact hole, but also when TiN is applied to the insulation layer with through-holes, when multilayer wiring that connects upper and lower wiring layers is applied. Furthermore, the same unavoidable problems occur even when wiring is drawn around over the insulation film.
Even in wiring that has a stacked structure other than the aforementioned TiN/Al stacked film, the same problems may occur. A stacked film assembly with a barrier function and sufficient EM resistance with good film formation and that can be formed easily has been desirable.
It is an object of this invention to provide a structure and method where crystal orientation of the top film can be satisfactorily controlled, and with which a stacked film assembly with sufficient barrier function and EM resistance can be formed satisfactorily and easily, by controlling the properties of the underlying film from a perspective different from that of the prior art.